3GPP long term evolution LTE technology aims to achieve higher data rate, shorter delay, less cost, higher system capacity and improved coverage scope. Multi-input multi-output MIMO technology is a crucial technology in enhancement of frequency spectrum efficiency.
In the MIMO wireless communication system, a transmitter and a receiver both use an antenna array, thereby providing rich space diversity and large communication capacity. Space multiplexing is a common space-time modulating technology for use in the MIMO communication system, wherein independent data streams are transmitted through different transmitting antennas.
In the LTE-A, the uplink supports single user MIMO (SU-MIMO), i.e., the uplink supports transmission of a plurality of antennas. This requires to provide a plurality of uplink UL demodulation reference signals (DM-RS) for all the space layers multiplexed together so that channel estimation can be carried out for each layer at a receiving end. The reference signal RS, as commonly called “pilot signal”, is a known signal provided by a transmitting end to the receiving end for channel estimation, synchronization or channel detection. Just as the technical term implies, the uplink UL demodulation reference signal DM-RS is used as a reference for data demodulation, whereby estimation is carried out with respect to the channel parameters, such as the phase and the amplitude by using the UL DM-RS, and thereby data transmitted on uplink can be demodulated correctly.
At the 3GPP RAN1 #57 meeting, the following DM-RS multiplexing principles are agreed in respect of supporting the uplink space multiplexing.                performing different cyclic shifts CSs for a pilot symbol serves as a main multiplexing mechanism;        multiplying different orthogonal cover codes OCCs between different time slots of the same data frame serves a complementary multiplexing mechanism.        
All the uplink DM-RSs have a reference signal sequence in the same form. The uplink DM-RS sequences in the LTE system can be defined by a base sequence plus cyclic shifts. Different amount of cyclic shifts is used for a base sequence, and a plurality of reference signal sequences can be defined.
An optimal orthogonality can be provided between different reference signals RSs by using the CS together with the OCC for DM-RS multiplexing, thereby providing an optimal performance.
However, space multiplexing is extremely sensitive to bad conditions of the channel. Hence, a hybrid automatic retransmission request HARQ mechanism is used to ensure correctness of transmission. The HARQ can be classified into two types, namely, adaptive retransmission and non-adaptive retransmission, depending on whether data characteristics upon retransmission change. The data characteristics comprise allocation of resource block, modulation mode, length of transmission block, duration of transmission and so on.
The adaptive retransmission means that in each retransmission procedure, the transmitting end can change partial transmission parameters according to actual channel state information, so support from relevant control signaling is needed.
In the non-adaptive retransmission, these transmission parameters are already known in advance to the receiving end, that is, the transmitting end and the receiving end are informed before the initial transmission. Hence, the non-adaptive system does not need the support of the corresponding control signaling.
Because of the complexity in the uplink, and interference from users in other cells is uncertain, the base station cannot accurately estimate actual SINR value of each user. Therefore, the 3GPP LTE system permits use of the non-adaptive HARQ technology in the uplink.
In the current LTE-A standardization procedure, the non-adaptive HARQ technology in the uplink is still under discussion and study. To date, there is not yet provided a corresponding solution about how to configure the DM-RS(s) when the user equipment UE carries out retransmission in the uplink.